How can I get rid of a virus that is on my website?

When I go onto my website - AVG tells me there's a threat - when I go to my webpage on my computer where the infection is detected, AVG cannot remove it as it can't be found although it comes up with the warning each time.

Its a Downloader Agent and also a Trojan Horse. It doesn't seem to be on my computer although it is showing that it is in my documents and settings. temporary files/contents. etc.

I don't know whether I can simply delete the file that it is pointing to.

To see if I could get rid of it by AVG deleting it or healing it, I tried to download the infected file onto my computer but AVG wouldn't allow it - so how do I get rid of it on my website?

I have been trying for a long time now.

Can anyone help?